1. Technical Field
The present disclosure relates to a photomask used in a process for manufacturing semiconductor devices, and more particularly, to a photomask having a reference pattern for creating a defect recognition level.
2. Discussion of Related Art
A photomask is a high precision plate containing patterns, i.e., microscopic images of electronic circuits. The photomask is used in wafer fabrication, mostly to make Integrated Circuits (ICs). Photolithography involves projecting photomask patterns onto a wafer. A defect is any flaw affecting the patterns on the photomask. Generally, defects on the photomask create errors to the ICs. The errors to the ICs may cause failures of a semiconductor device. Therefore, there exists a need to inspect the defects of the photomask during manufacturing semiconductor devices.
Conventionally, when inspecting for defects, a pattern being inspected is compared to a pattern on a database. With this inspection method, an image seen through a known inspecting device is compared to a digitalized image on the database. In other words, inspection results, obtained by detecting light-shielding patterns on the photomask, are compared to a reference data on the database. Another conventional inspection method is to compare two identical patterns after forming the two identical patterns on one photomask to determine if there is any discrepancy. The inspection results are also used to compare the two identical patterns. In the conventional inspecting methods, it is known that a defect recognition level is used to obtain the inspection results.
FIG. 1A is a plain view showing a reference pattern for creating a conventional defect recognition level. FIG. 1B is a cross-sectional view taken along the line I-I shown in FIG. 1A. Referring to FIGS. 1A and 1B, a conventional reference pattern includes a blank region A in which a transparent substrate 10 is exposed, and a light-shielding region B in which a light-shielding layer 20 is formed on the transparent substrate 10. The light-shielding region B is formed adjacent to the blank region A.
FIG. 1C is a graph showing a defect recognition level obtained by using the conventional reference pattern. Referring to FIG. 1C, light (not shown) is irradiated on an area including the blank region A and the light-shielding region B shown in FIG. 1A. Then, intensities of light transmitted to the blank region A and the light-shielding region B are measured by a known device. Most of the light is transmitted through the blank region A. The intensity of the light transmitted to the light-shielding region B is lower than the intensity of light transmitted to the blank region A because most of light irradiated on the light-shielding region B is blocked by the light-shielding layer 20.
Then, as shown in the graph of FIG. 1C, the intensity of the light transmitted to the blank region A, and the intensity of the light transmitted to the light-shielding region B can be indicated by normalized intensity values, respectively. For example, the intensity of the light transmitted to the blank region A is determined as a standard intensity value “1”. The intensity of the light transmitted to the light-shielding region B can be represented as a relative value to the standard intensity value “1”. A middle value between the standard intensity value “1” and the normalized intensity value of the light transmitted to the light shielding region B is determined as a defect recognition level 30.
After the defect recognition level 30 is determined, light is irradiated on an inspection pattern (for example, a pattern shown in FIG. 1D) of a photomask which is to be inspected. Then, intensities of light transmitted to the inspection pattern are measured by a known device. If a normalized intensity value of the light transmitted to the inspection pattern is lower than the defect recognition level 30, it is recognized that a light-shielding pattern exists on the transparent substrate.
FIG. 1D is a cross-sectional view of an inspection pattern of an alternating phase shift photomask under a defect inspection. Referring to FIG. 1D, the alternating phase shift photomask 400 includes a phase 0 degree region 21, a light-shielding region 23, and a phase shift region 25. A transparent substrate 11 is exposed in the phase 0 degree region 21. The transparent substrate 11 is covered with a light-shielding layer 22 in the light-shielding region 23. The transparent substrate 11 is etched to form the phase shift region 25. The phase 0 degree region, the light-shielding region 23, and the phase shift region 25 can alternately be aligned. For example, in a Cr-less mask (CLM), the phase 0 degree region 21 and the phase shift region 25 are alternately aligned because the CLM does not include a light-shielding layer 23.
When the transparent substrate 11 is etched to form the phase shift region 25, a transparent defect 27 can be formed in the phase shift region 25. Generally, because the transparent defect 27 (which was a part of the transparent substrate) is formed of the same material as the transparent substrate 11, the transmittance of the transparent defect 27 is same as the transmittance of the transparent substrate 11.
FIG. 1E is a graph showing a relationship between the defect recognition level 30 and normalized intensity values of light transmitted to the inspection pattern of an alternating phase shift photomask 400 (shown in FIG. 1D). Referring to FIG. 1E, after determining the defect recognition level 30, light (not shown) is irradiated on the inspection pattern of the phase shift photomask 400 under the defect inspection. Then, intensities of the light transmitted to the inspection pattern are measured by a known device. When the normalized intensity value of light is close to “1”, it is recognized that there is no light-shielding pattern (for example, the light shielding pattern 22 shown in FIG. 1D).
When the normalized intensity value of light is close to “0”, it is recognized that there exists a light-shielding pattern. Because most of light can be transmitted through the transparent defect 27, the normalized intensity value of light transmitted to the transparent defect 27 is close to “1”. However, the light transmitted to the phase shift region 25 and the light transmitted to the transparent defect 27 interfere with each other so that the normalized intensity value of light measured at a boundary of the transparent defect 27 and the phase shift region 25 is lower than “1”.
The intensity of the light measured at the boundary of the transparent defect 27 and the phase shift region 25 varies depending on a phase angle of the light transmitted to the transparent defect 27. The intensity of the light provides various values of a defect distribution 35 depending on thickness of the transparent defect 27.
The lowest normalized intensity value of light in the defect distribution 35 can be obtained when the intensity of the light transmitted to the transparent defect 27 and the intensity of the light transmitted to the phase shift region 25 have 180 degrees of phase difference.
Even the lowest normalized intensity value in the defect distribution 35 can have a higher normalized intensity value than the normalized intensity value of the light measured at the light-shielding region 23 due to a limitation of a known inspection device. Because the intensity of light measured by the known inspection device is based on a given area, it is difficult to measure the intensity of light at the boundary of the transparent defect 27 and the phase shift region 25. As a result, when measuring the intensity of the light at the transparent defect 27 and the phase shift region 25, an intensity of light transmitted to adjacent areas can be included. Therefore, most of the values of the defect distribution 35 caused by the transparent defect 27 are located above the defect recognition level 30. As a result, most of the transparent defects 27 cannot be detected by the conventional defect recognition level 30.